depth of field to work not working
Depth of field (as defined by circles of confusion) depends on three factors:


The length of the lens used (in mm). Longer lenses will have less depth of field
The distance to the subject. The closer the camera is to the object the more critical focus will be, and the depth of field will be shallower.
The f stop on the lens. Higher f-stops will give you larger depth of field (more things will be in focus) Smaller f-stops will reduce the depth of field.


You are using a wide angle lens (35mm) and a very high  number for f stop (31.8), and your object are quite large and far away.  with those settings is very unlikely that you will see the effects of selective focus (which I presume is what you are looking for). Lower the F-stop to something like 1.4 or even less than 1 to see the effects. Also in cycles you can preview the depth of field in preview render mode.